THE LIVING WORLD
Unit Four. The Evolution and Diversity of Life
The phylum Chytridiomycota (chytrids) are mostly aquatic organisms, although some are found in soils and other terrestrial environments. Chytrids are the most primitive fungi, retaining flagellated gametes (called zoospores) from their protist ancestors. The other fungal groups are thought to have lost their flagellated stage at some point early in their evolutionary history as fungi. Several species of chytrids are plant pathogens that cause minor diseases. One species of chytrid, Batrachochytrium dendrobatidis, has been identified as a potential pathogen of frogs. The spores released by the fungi become embedded in the skin, where they seem to interfere with normal skin functions like respiration. A reddening of the skin on the legs and abdomen of the infected frog in figure 18.9 is a symptom of a B. dendrobatidis infection. This fatal disease appears to be a contributing cause to the recent sharp declines in amphibian populations seen worldwide.
Figure 18.9. Chytrid infection.
The pathogenic chytrid, Batrachochytrium dendrobatidis, has infected this frog.
In addition to the four phyla of fungi that differ primarily in their mode of sexual reproduction, there are some 17,000 described species of fungi in whom sexual reproduction has not been observed. These cannot be formally assigned to one of the four sexually reproducing phyla and so are grouped for convenience as the so-called imperfect fungi (figure 18.10). The imperfect fungi are fungi that have lost the ability to reproduce sexually. Most of them appear to be ascomycetes, although some basidiomycetes are also included—these can be distinguished by features of the hyphae and asexual reproduction. Most of the fungi that cause skin diseases, including athlete’s foot and ringworm, are caused by imperfect fungi. Fungal diseases are often difficult to treat pharmaceutically because of the ancestral relationship between animals and fungi. Medicines that kill fungal cells may also adversely affect animal cells.
Figure 18.10. Imperfect fungi.
Imperfect fungi are fungi in which sexual reproduction is unknown. Verticillium alboatrum, an important pathogen of alfalfa, has whorled conidia. The single-celled conidia of this member of the imperfect fungi are borne at the ends of the conidiophores.
Yeast is the generic (general) name given to any unicellular fungus. Although single-celled, yeasts appear almost certainly to have been derived from multicellular ancestors. There are about 250 named species of yeasts, including Saccharomyces cerevi- siae (brewer’s yeast), used for thousands of years in the production of bread, beer, and wine. Other yeasts are pathogens, including Candida, a common source of vaginal infection.
Just as in ascomycetes, most of yeast reproduction is asexual and takes place by cell fission or budding (the formation of a small cell from a portion of a larger one, as you see happening at the arrow in figure 18.11). Sexual reproduction among yeasts occurs when two yeast cells fuse. The new cell containing two nuclei functions as an ascus. After the two nuclei fuse, meiosis produces four ascospores, which develop directly into new yeast cells.
Figure 18.11. Budding in Saccharomyces.
These yeast cells tend to hang together in chains, a feature that calls to mind the derivation of single-celled yeast from multicellular ancestors.
Key Learning Outcome 18.8. Chytridiomycetes are a group of fungi most closely related to ancestral fungi. Imperfect fungi may have lost the ability to reproduce sexually. Yeasts are unicellular fungi.